1. Field of the Invention
The invention relates to a vehicle steering system that includes a motor control device for driving a three-phase brushless motor.
2. Discussion of Background
A drive circuit for a brushless motor used in an electric power steering system includes switching elements, such as field effect transistors (FETs). If a fault occurs in one of the switching elements, there is a possibility that the brushless motor becomes a load when a steering wheel is operated and, as a result, steering operation becomes heavy. In order to cope with such a problem, relays are provided on connecting lines between the brushless motor and the drive circuit. For example, US2009/0251831 A1 describes that relays are provided on respective motor connecting lines for two phases of a three-phase brushless motor and those relays are turned off when the motor is not controlled or when a fault occurs in one of switching elements.
However, with the technique described in US2009/0251831 A1, it is not possible to identify a short-circuited switching element.